In full-duplex communications over a two-conductor transmission line, it is desirable to remove from a composite receive signal that portion due to reflection of a local transmit (Tx) signal, to thereby yield a corrected receive (Rx) signal which closely approximates the true signal broadcasted from a remote transmitter. Such correction is generally accomplished using a signal separator, or "hybrid," and a subtractor.
FIG. 1 shows an abbreviated electrical block diagram of a telecommunications system 100, which includes a telecommunications apparatus 102 operative to communicate with a telecommunications apparatus 104 over a transmission line 106. Telecommunications apparatus 102, which may be a modem, typically includes a transceiver having a transmitter 108 and a receiver 110, a conventional hybrid circuit 112, a subtractor 114, and a line interface 116. Telecommunications apparatus 104 includes the same or similar components, at least a transceiver with a transmitter 118 and a receiver 120 coupled to a line interface 122.
Conventional hybrid circuit 112 typically includes a conventional (2-port) balancing network 124, a transmit source impedance (Z.sub.S) 126, which is typically resistive, and a transmission line input impedance (Z.sub.i). Circuitry of FIG. 1 is diagramed in an unbalanced (or ground-referenced) form for simplification, although balanced (or ungrounded) circuitry is frequently employed in practice.
Conventional balancing network 124 seeks to develop a voltage V.sub.B which is equal to, in magnitude and phase over the frequency range of interest, a reflected transmit signal component of voltage V.sub.A. Thus, a subtractor 114 may yield a subtractor output voltage V.sub.C =V.sub.A -V.sub.B which is equal to the incoming receive signal. For this to work, the voltage transfer function V.sub.B /V.sub.Tx of conventional balancing network 124 must be equal to that of the Z.sub.S -to-Z.sub.i voltage divider (Z.sub.i /Z.sub.S +Z.sub.i). This implies that conventional balancing network 124 must include elements that emulate the magnitude and phase variation behavior of Z.sub.i over the frequency range of interest.
Typically, transmission line 106 is a twisted pair of insulated conductors whose impedance magnitude as a function of frequency behaves as shown in a graph 500 of FIG. 5. More particularly, graph 500 is representative of a twisted pair of insulated copper wires having AWG gauges 19 through 26, typically used in cables for telephony, Integrated Services Digital Network (ISDN), Digital Subscriber Lines (xDSL), and related communication formats. A line impedance curve 502 (shown in solid) represents a long (e.g., 10,000 feet (3048 meters)), unimpaired, terminated transmission line. A line impedance curve 504 (shown in dotted) represents a transmission line impaired by a long bridged tap located near the line's input. These bridged taps are often installed on the main lines in anticipation of line sharing and are typically inaccessible and of unknown length. The phase behavior of such transmission lines is closely and predictably related to the impedance-magnitude behavior and, for brevity, is not separately shown. Conventional hybrids have been designed to accommodate these monotonically-decreasing impedance versus frequency behaviors at low frequencies, such as voiceband frequencies (generally about 300-3000 hertz (Hz) and low data rate communication frequencies (tens of kilohertz (kHz) and below, as in modems operative at 14.4 through 56 kilobits per second (kbps)).
FIGS. 2, 3, and 4 show various conventional balancing networks 200, 300, and 400, respectively, for use in conventional hybrid circuit 112 of FIG. 1. Each of conventional balancing networks 200, 300, and 400 includes an impedance structure 202 and an impedance structure 204, with nodes 128 and 130 for coupling within conventional hybrid circuit 112 as indicated in FIG. 1.
Conventional balancing network 300 of FIG. 3 provides a fixed one-pole, one-zero transfer function, and has been proposed for 784 kbps 2B1Q High-data-rate Digital Subscriber Lines (HDSL) transceivers where the frequencies of primary interest extend to about 200 KHz. See "Generic Requirements for High-Bit-Rate Digital Subscriber Lines," Bellcore Technical Advisory TA-NWT-001210, Issue 1, October 1991; and W. Y. Chen et al., "High Bit Rate Digital Subscriber Line Echo Cancellation," IEEE Journal on Selected Areas in Communications, Vol. 9, No. 6, August 1991. Conventional balancing network 400 of FIG. 4 implies adjustability of these pole and zero locations, which is described in detail in U.S. Pat. No. 4,096,362 (Crawford). Conventional balancing network 400 may include a magnitude-scaling component to accommodate the presence of bridged-tap line impairments, since the low frequency impact of such impairments simply uniformly scales the impedance magnitude over frequency. Multiple-pole, multiple-zero networks are also available to provide an arbitrarily close match to a given monotonically-decreasing impedance characteristic. Analysis and measurements show that such conventional balancing networks, whether fixedly or adaptively configured, are somewhat useful for unimpaired lines or for impaired lines with long bridged taps.
When one or more short bridged taps are present near a transmission line input, however, a conventional hybrid with one of these conventional balancing networks is found to be highly inadequate--or even counterproductive. In this context, a short bridged tap is one whose length is between roughly one-sixteenth wavelength at the maximum frequency of interest and several thousand feet. For example, short tap lengths would range from about two hundred to several thousand feet for 784-kbps HDSL.
FIG. 6 is a diagram which illustrates such a transmission line environment. A bridged tap 604 is tapped between transmission line sections 602 and 606 of a main line terminated by an impedance 608. Here, bridged tap 604 has a length Y.sub.2 located at distance Y.sub.1 from the line's input. A distance Y.sub.T =Y.sub.1 +Y.sub.2 is the total distance from the line input to the end of the tap. While only a single local tap is shown, multiple local taps may also be present in such an environment.
FIG. 7 is a graph 700 showing some line impedance versus frequency behaviors of the environment shown in FIG. 6. The numerical impedance values shown in graph 700 represent AWG #26 twisted-pair lines. A line impedance curve 702 (shown in solid) represents the behavior of a long unimpaired line; a line impedance curve 704 (shown in dotted) represents the behavior of a long line having a bridged tap of length 800 feet (244 meters) near the line's input; and a line impedance curve 706 (shown in dashed) represents a long line having a bridged tap of length 1600 feet (488 meters) near the line's input. These tap-impaired impedance behaviors are non-monotonic and oscillatory, and are not adequately emulated by conventional hybrids. The frequencies of impedance minima and maxima depend on tap length. Thus, a conventional hybrid designed for an 800-foot tap would badly mismatch a 1600-foot tap.
FIG. 8 is a graph 800 revealing a further complication in impedance matching. Line impedance curves 802, 804, and 806 of FIG. 8 each correspond to a combined distance of 800 feet (244 meters) from the input to the end of the tap (distance Y.sub.T in the preceding discussion), but with different tap lengths and locations: line impedance curve 802 (shown in solid) represents an 800-foot (244-meter) tap at the input; line impedance curve 804 (shown in dotted) represents a 600-foot (183-meter) tap located 200 feet (61 meters) from the input; and line impedance curve 806 (shown in dashed) represents a 400-foot (122-meter) tap located 400 feet (122 meters) from the input. While the frequencies of the impedance extremes are essentially the same, the extreme impedance values are not. Further analysis and measurements have shown this to be generally true for all bridged tap lengths and locations of interest.
Accordingly, what is needed is a hybrid to accommodate these and similar problems.